Method of making anodes for hydrazine fuel cells

ABSTRACT

A method of making nickel boride catalyzed anodes for hydrazine fuel cells including the step of co-depositing electrolytic nickel and carbonyl nickel particles onto an appropriate conductive substrate to form a catalyst-receiving surface. Sufficient electtolytic nickel is deposited to tack the carbonyl nickel particles to the substrate and to themselves, yet not so much as to appreciably degrade the characteristic rough and jagged surfaces of the carbonyl nickel particles.

United States Patent [191 Zeitner, Jr. et a1.

[ Nov. 13, 1973 METHOD OF MAKING ANODES FOR HYDRAZINE FUEL CELLS [75]Inventors: Edward J. Zeitner, Jr., Sterling Heights; Marion E. Wheatley,Warren; Romeo R. Witherspoon, Utica; Stuart G. Meibuhr, Birmingham, allof Mich.

[73] Assignee: General Motors Corporation,

Detroit, Mich.

[22] Filed: July 17, 1972 [21] Appl. N0.: 272,309

[52] US. Cl. 136/120 FC [51] Int. Cl. H0lm 27/04 [58] Field of Search136/129 PC, 120,

[56] References Cited UNITED STATES PATENTS 3,183,123 5/1965 Haworth136/120 FC X 3,380,856 4/1968 Pohl 136/120 FC 3,437,526 4/1969 Lindholmet a1, 136/120 FC X 3,513,028 5/1970 Salomon 136/120 FC X PrimaryExaminerAnthony Skapars AttorneyWilliam S. Pettigrew [57] ABSTRACT 4Claims, 2 Drawing Figures METHOD OF MAKING ANODES FOR HYDRAZINE FUELCELLS This invention relates to nickel boride catalyzed anodes forhydrazine fuel cells and more particularly to a method of making anodeswhich retain the nickel boride during the rigors of cell discharge.Nickel boride has been proposed as a catalyst for a number of reactionsincluding the oxidation of hydrazine in fuel cells such as thehydrazine-air or hydrazine-oxygen cells. The term fuel cell is usedherein to mean a current generating electrochemical cell in which fuelis continuously consumed at an anode and oxidant continuously consumedat a cathode with an appropriate iontransporting electrolyte between andin contact with both. Closing an external circuit between the anode andcathode permits the withdrawal of useful electrical energy from thecell. In hydrazine-fueled cells, the hydrazine fuel is usually dissolvedin an electrolyte, such as potassium hydroxide (KOl-l) and flows incontact with the anode, which comprises a suitable conductive supportcarrying a catalyst (i.e., nickel boride). The hydrazine-KOH solutionforms the cells anolyte. The cathode contacts a catholyte which, in thecase of an air or oxygen cathode, comprises a potassium hydroxidesolution without any dissolved hydrazine. The anolyte and catholyte areseparated one from the other by a suitable membrane for impedinghydrazine transfer from the anolyte to the catholyte without impairingionic mobility between anode and cathode.

One of the problems with nickel boride catalyzed hydrazine anodes is ofshort useful life especially at high anode current densities. A majorcontributor to the short useful life of hydrazine anodes is the violentoxidation reaction occurring at the anode which forms nitrogen inmicroexplosions adjacent the catalyst, the repeated occurrence of whichbatters and destroys the electrodes by dislodging the catalyst from theconductive support.

It is an object of this invention to provide a process for making a highsurface area, conductive support capable of providing so firm ananchorage for the nickel boride catalyst that it is not so readilydislodged from the support during cell discharge whereby the anodes lifeis significantly increased. This and other objects and advantages ofthis invention will become more apparent from the detailed descriptionwhich follows.

This invention, comprehends a process for making hydrazine anodes andmore particularly a process for forming a catalyst-receiving surface onthe conductive supporting member of the anode. The conductive supportingmember may be formed of any conductive material, which is chemicallyresistant to the corrosive environment of the electrolyte and which hasa large surface area for contacting the anolyte. In this regard, themacro-structure of the support should be such as to consume very littlecell volume while still presenting a considerable amount of surface areato the anolyte flowing over or through it. For flow-through celldesigns, such a support then could be made from open cell reticulatedmetal foam, wire mesh and/or loose felts of sintered metal fibers. Forflow-over cell designs, impervious scrobiculated metal foils havingdimpled or grooved surfaces are particularly desirable. Compositionwise, stainless steel, carbon or the like may be used without concernfor corrosion, but nickel is preferred since it not only resistscorrosive attack but is easy to fabricate and has a low resistivity.

The surface of the support is treated to provide a plurality of firmlybonded anchorage sites for receiving and holding nickel boride catalyst.To this end, the conductive support is made the cathode in a nickelelectroplating bath containing carbonyl nickel particles suspendedtherein. As electrolyzing current passes through the electroplatingcell, electrolytic nickel is deposited onto the surface of the supportand carries with it some of the suspended particles. Hence the carbonylnickel particles are co-deposited along with the electeolytic nickelwhich electrolytic nickel acts as a matrix, firmly tacking and holdingthe carbonyl nickel particles onto the surface of the support.

Carbonyl nickel particles which are produced by the decomposition ofnickel carbonyl, vary in size from about 2 to about 10 microns and havea distinctive or characteristic rough and jagged surface as seen in V.A. Tracey and N. J. Williams, Electrochemical Technology, Jan.-Feb.,1965. It is important that the rough surface of the carbonyl nickel beretained and hence not degraded during electrolysis of the nickel bath.it is essential that only enough electrolytic nickel be deposited tofirmly bond the carbonyl nickel particles to the support. Should toomuch electrolytic nickel be deposited, the particles, which deposittherewith, become coated and their characteristic rough surface is lostto the point where eventually they no longer provide the excellentnickel boride anchorage sites contemplated by this invention. Morespecifically, the niclel equivalent of at least 5 coulombs per squarecentimeter of support surface should be deposited'to insure sufficienttacking of the particles to the support. At the other extreme, it hasbeen noted that when more than about 24 coulombs per square centimeterof support surface is used the rough texture was lost and the nickelboride quickly shed in the cell. It is not necessary to have aparticularly thick layer of the nickel-bound particles so long as theyare firmly bound in the electrolytic nickel and are spread uniformlyacross the face. In this regard, no particular benefits are achievedabove a preferred loading of about 10 milligrams of carbonyl nickel persquare centimeter of support surface. Hence, though more can be used, itis uneconomical to do so. On the other hand, at least about twomilligrams of carbonyl nickel per square centimeter of support surfaceis required to insure adequate take up and anchorage of the nickelboride catalyst.

The benefits of this invention are available regardless of theparticular nickel plating bath used and hence any of the conventionallow or high stress baths known in the art may be employed for obtainingthe electrolytic nickel matrix. In a preferred form of the invention,the carbonyl nickel particles are suspended by stirring in a nickelsulfamate bath, the support is positioned horizontally near the bottomof the plating cell and the stirring discontinued after about the first10 percent (10 percent) of the plating is completed leaving theparticles to settle out or gravitate onto the support during theremainder of the plating cycle. The plating time itself should beextended to at least 2 minutes and preferable about five minutes or moreafter the stirring has stopped to insure adequate settling of theparticles into the support.

The thusly prepared substrate is next catalyzed by forming nickel boridein situ on the nickel-bound carbonyl nickel particles. This should bedone using formation techniques in which the nickel boride forms belowabout 400C, since it has been observed that the carbonyl nickelparticles tend to sinter or coalesce resulting in a loss of their rough,jagged surface when exposed to temperatures above about 400C. for anysubstantial period of time. It is preferred to form the nickel boride onthe support by the known process of first immersing the support into asolution of a nickel salt and subsequently immersing it into a solutionof an alkali metal borohydride which converts the nickel salt to nickelboride and forms a corresponding alkali metal salt and hydrogen. Severalrepetitions of this process provides a sufficient amount of nickelboride on the surface. The aforesaid immersion process is essentially aroom temperature reaction having no control problems and results in anintimate engagement between the nickel boride and the nickel-boundcarbonyl nickel particles. The solvent for the nickel salt and alkalimetal borohydride is not particularly significant and accordingly mayeither be water or an organic, so long as it will dissolve a sufficientamount of the salt or borohydride as to provide a practically usefulconcentration. Aqueous solutions are preferred for both reactants, butalcohol may be used for the nickel salts and alcohol and the dimethylether of diethylene glycol may be used for the borohydride (e.g., sodiumborohydride). Among the soluble, reducible nickel salts that can be usedhere are the chlorides, acetates, bromides, nitrates, and mixturesthereof, with the acetate being preferred. Useful borohydrides can betaken from the alkali metal group of sodium, potassium, lithium, cesium,and rubidium borohydrides, but as a practical matter, only the lithium,sodium and potassium borohydrides need be considered since they arecommercially available. One of the principal advantages of using thealkali metal borohydrides is the fact that the alkali metal salt formedfrom the anion of the nickel salt is much more soluble than the nickelboride formed and hence removal of all of the reaction products but thenickel boride is readily accomplished by mere rinsing in water.

As an example of the invention, dimpled nickel foils (0.05 mm thick)were electroformed on a chromiumplated, stainless steel mandrel from an18 liter nickel sulfamate and nickel chloride bath comprising 300 gramsper liter nickel sulfamate, 6 grams per liter nickel chloride and 30grams per liter boric acid (pl-I 2.3 4.0). The geometric area of thedimpled foil was 48 cm. Carbonyl nickel particles (Inter-National NickelCo.) were then electrolytically co-deposited onto the electroformedfoils using a vertical half-box arrangement with the foils at the bottomand using 500 ml of the aforesaid nickel sulfamate electroformingsolution and 500 milligrams of carbonyl nickel. A nickel anode screenwas spaced above the foil to provide an electrode gap of 62 mm. Apotential was established between the foil and the screen and the nickelbath added to the cell. The stirrer was energized followed by theaddition of the carbonyl nickel. Stirring continued for 1 minute andthen stopped. Plating continued for about 9 minutes after stirringstopped for a total of about 10 minutes total plating time at a currentdensity of about 0.04 amps/cm of the supports geometrical area. In othertests, a high-stress (300 g/l Nicl nickel plating bath was successfullyused to co-deposit the carbonyl powder.

Two carbonyl Ni powders were used in this study International NickelMond 255 and Mond 128, and in co-deposition bath concentrations of about1 gram per liter. Lower powder concentrations are useable but theplating time must then be extended to insure an adequate build-up ofparticles on the surface. Similarly, higher powder concentrations may beused but should not be so high as to produce so high a settling ratethat the electrolytic nickel will not adequately bind the particles at agiven plating rate. Their particle sizes were approximately 3 and 8micrometers for the Mond 255 and Mond 128 powders, respectively. Sampleswere prepared over the range of about 5 coulornbs/cm to about 72coulombs/cm with the results discussed above. Various stirring speedswere used from 1,000 to 10,000 rpm, but there was virtually no effect ofstirring speed on the surface structure and texture of these substrates.Hence, for most of the polarization stuides the substrates were preparedat 7,100 rpm.

The Ni B catalyst was chemically formed in and on the carbonyl nickel byfirst dipping the support into a 5 aqueous solution of nickel acetatefor about 15 seconds, draining and then dipping it into an aqueous 10solution of sodium borohydride for about 30 seconds and rinsing. Thisprocedure was repeated three times and the anodes tested immediatelyafter the final rinse.

The catalyzed anodes (48 cm active area) were operated at a constantcurrent density of 200 mA/cm for about 2 hours before the initialiR-free polarization data was taken. Test temperature was 31 i 1 C.using a 33% KOH electrolyte containing 3.2% hydrazine and flowing overthe anode at a rate of about ml/min. At this flow rate, the fuelconcentration was about 16 times the stoichiometric amount. Perforatedstainless steel was used as the counter electrode at which hydrogen wasevolved during testing. Polarization data were made daily and the anodewas operated at 200 mA/cm" between polarization runs. The test wasterminated when the anode-reference potential (using a Hg/HgO referenceelectrode) had fallen to O.95V using a modified Kordesch-Markointerrupter to correct for solution iR-drop. The anodes were capable ofoperating for longer times at potential values below 0.95V, but thisvalue was selected as the cutoff potential for evaluation purposes. Theanolyte (as above) and catholyte (33% KOH) were separated by amicroporous styrene acrylonitrile membrane material known as Acropor AN200 sold by the Gelman Instrument Company.

The polarization plot shown in FIG. 1 is typical for practically all theNi B-catalyzed substrates at the beginning of the testing procedures.This plot is best represented by the spread as depicted by the shadedarea. Duplicate runs, while not giving exactly the same potentialvalues, did fall within the shaded area. The data in FIG. 2 portrays thetime dependence of the anode potential; the time scale represents onlythat time the anode operated at 200 mA/cm. The anode, however, remainedin contact with the KOI-I electrolyte for about four times greaternumber of hours than indicated by the operating times given in FIG. 2.

While we have disclosed our invention solely in terms of specificembodiments thereof we do not intend to be limited thereto, but ratheronly to the extent hereinafter set forth in the claims which follow.

We claim:

I. A method of making a fuel electrode for use in a hydrazine-fueledfuel cell including the steps of:

a. making an electrically conductive support the cathode in a nickelelectroplating cell;

b. filling the cell with a nickel plating bath containing suspendedparticles of carbonyl nickel;

c. passing at least about 5 coulombs per square centimeter of supportand less than 24 coulombs per square centimeter of support through saidcell to electrolyze said bath and plate electrolytic nickel onto thesupport while codepositing the particles onto the support along with theelectrolytic nickel such that there is sufficient electrolytic nickelplated to securely tack the particles to the support yet an insufficientamount to appreciably degrade the characteristic rough and jaggedsurface of the carbonyl nickel particles;

(1. contacting the support and wetting the nickelbound particles thereonwith a solution of a soluble nickel salt and solvent therefor; and

e. contacting said wetted nickel-bound particles with a solution of analkali metal borohydride and solvent therefor to form nickel boride insitu on the nickel-bound carbonyl nickel particles.

2. A method of making a fuel electrode for use in a hydrazinefueled fuelcell including the steps of:

a. making an electrically conductive support the cathode in a nickelelectroplating cell;

b. filling the cell with a nickel plating bath containing particles ofcarbonyl nickel;

c. suspending the particles in the bath above the support by rapidlystirring the bath for a predetermined time;

d. passing at least about 5 coulombs per square centimeter of supportand less than 24 coulombs per square centimeter of support through saidcell and over a time span which greatly exceeds said predetermined timeto electrolyze said bath and plate electrolytic nickel onto the support;

e. codepositing the particles onto the support along with theelectrolytic nickel during said predetermined time;

f. discontinuing said stirring after said predetermined time andallowing said particles to gravitate onto the support while continuingthe nickel plating over said time span to electrolytically depositsufficient electrolytic nickel onto the support to securely tack theparticles to the support without appreciably degrading thecharacteristic rough and jagged surface of the nickel particles;

g. contacting the support and wetting the nickelbound particles thereonwith a solution of a soluble nickel salt and solvent therefor; and

h. contacting said wetted nickel-bound particles with a solution of analkali metal borohydride and solvent therefor to form nickel boride insitu on the nickel-bound carbonyl nickel particles.

3. The process according to claim 2 in which said predetermined time isabout 10 percent ot the total plating time span and said time spanextends for at least about five minutes.

4. A hydrazine anode for a hydrazine-fueled fuel cell comprising anelectrically conductive support having at least about two milligrams ofcarbonyl nickel held in a matrix of electrolytic nickel for each squarecentimeter of said support and a nickel-boride catalyst formed in andfirmly anchored to the carbonyl nickel particles.

2. A method of making a fuel electrode for use in a hydrazine-fueledfuel cell including the steps of: a. making an electrically conductivesupport the cathode in a nickel electroplating cell; b. filling the cellwith a nickel plating bath containing particles of carbonyl nickel; c.suspending the particles in the bath above the support by rapidlystirring the bath for a predetermined time; d. passing at least about 5coulombs per square centimeter of support and less than 24 coulombs persquare centimeter of support through said cell and over a time spanwhich greatly exceeds said predetermined time to electrolyze said bathand plate electrolytic nickel onto the support; e. codepositing theparticles onto the support along with the electrolytic nickel duringsaid predetermined time; f. discontinuing said stirring after saidpredetermined time and allowing said particles to gravitate onto thesupport while continuing the nickel plating over said time span toelectrolytically deposit sufficient electrolytic nickel onto the supportto securely tack the particles to the support without appreciablydegrading the characteristic rough and jagged surface of the nickelparticles; g. contacting the support and wetting the nickel-boundparticles thereon with a solution of a soluble nickel salt and solventtherefor; and h. contacting said wetted nickel-bound particles with asolution of an alkali metal borohydride and solvent therefor to formnickel boride in situ on the nickel-bound carbonyl nickel particles. 3.The process according to claim 2 in which said predetermined time isabout 10 percent ot the total plating time span and said time spanextends for at least about five minutes.
 4. A hydrazine anode for ahydrazine-fueled fuel cell comprising an electrically conductive supporthaving at least about two milligrams of carbonyl nickel held in a matrixof electrolytic nickel for each square centimeter of said support and anickel-boride catalyst formed in and firmly anchored to the carbonylnickel particles.